C3 Deficiencies

Abstract

The third component of complement is central to both pathways of the complement cascade and in mediating or initiating the bulk of physiological and often pathological effects of this system; it is also dominant due to its abundant plasma concentration and the number of different epitopes and binding sites for a large number of cooperating and regulatory, soluble and membrane-bound proteins. Consequently, deficiencies in C3 would be expected to exhibit a life-threatening clinical picture. In the early years of discovering complement deficiency states it was thought that C3 deficiency in man does not exist because of the presumed inherent lethality. When the first human C3 deficiences were described, followed by reports of genetic defects of C3 in guinea pigs and dogs, a more realistic and differentiated picture as to the biological role of C3 in vivo emerged. This view was supported by in vivo data from transient C3 deficiencies secondary to massive activation of C3. Our knowledge about the central function of this component has been broadened through experimental in vivo models of C3 depletion and disease-associated C3 consumption or hypercatabolism, due to disturbances in the regulatory network which physiologically guarantees a delicate balance between activation and inactivation of C3.